1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter abbreviated as TFT) with a noncrystalline semiconductor film.
2. Description of Related Art
As substitute for a conventional TFT with a single crystalline semiconductor, there has been developed a TFT with a noncrystalline semiconductor, such as an amorphous semiconductor, a polycrystalline semiconductor, or a mixed crystalline semiconductor of the both.
Among such noncrystalline semiconductors, the most suitable material for forming a lot of TFTs on a large area substrate is amorphous semiconductor material, especially amorphous silicon (hereinafter abbreviated as a-Si) because it has the advantage that its semiconductor properties as a transistor is stable and that it can be formed by plasma CVD (chemical vapor deposition) which is able to form a large area film.
Recently, such an a-Si TFT as mentioned above has been employed and put to practical use for each transistor on a switching transistor array substrate of an active matrix type liquid crystal display unit.
Such a TFT has both advantages that a large area semiconductor film can readily be formed by plasma reaction as mentioned above and that using this plasma reaction enables to continuously form a silicon nitride (SiNx) film or a silicon oxide (SiO.sub.2) film which is to be a gate insulating film or a passivation film of a TFT only by changing reaction gases.
However, the electron mobility of such a-Si TFT is merely 0.5 cm.sup.2 /V.s restricted by the quality of a-Si film, then, enough electric current for driving liquid crystals has not obtained.
In recent years, in order to obtain enough electric current for driving the liquid crystal, a polycrystalline silicon (p-Si) TFT with high electron mobility has been employed, as a result, an active matrix type liquid crystal display unit, which can shorten the switching time of the TFT with higher quality of display, has been put to practical use.
Such a p-Si TFT is manufactured by forming p-Si films by LPCVD method or by recrystallizing the a-Si film later in this process. As for recrystallizing the a-Si film, in addition to heat annealing, there has been developed laser annealing for obtaining a local polycrystalline film by irradiating high energy laser beams.
Where manufacturing the p-Si film of the p-Si TFT by of locally annealing the a-Si film with the laser beams, the p-Si film can be obtained simply by irradiating the laser beams. However, this local annealing results in generating stresses between the p-Si film and another thin film constituting the TFT. As a result, in wet etching, abnormal etching may possibly break a designed pattern of the TFT.
As shown in FIG. 1, in the conventional laser annealing as mentioned above, a great amount of stresses are generated in outline boundaries 19, 19 . . . of the pattern between a polycrystalline film 6 and a gate insulating film which is coated over the whole surface of this polycrystalline film 6, as a result, wet etchant which soaks into the outline boundaries should generate inconvenient abnormal overetching. It is considered that the stresses which generally exist in between thin films concentrate on the outline boundaries 19, 19 . . . of the pattern.
Though such a polycrystalline silicon TFT can shorten the switching time, there is the disadvantage that the OFF-state current is larger. Then, as a general single crystalline silicon FET, impurities are introduced into a source and drain area to make the conductivity of the source and drain area different from that of a channel area so as to form PN junction between these two areas.
However, this experiment still can not substantially reduce the OFF-state current, then, a practical TFT is not obtained.
The conventional active matrix type liquid crystal display unit, particularly what is called a liquid crystal television set displays images by using the operation of storing electric charge of the liquid crystal, then, the stored electric charge in liquid crystals charged in every frame should be held for a period of displaying one frame. As a result, the holding voltage of the liquid crystals is reduced because of the current leakage during that period which deteriorates the quality of display.
Though a transparent liquid crystal television set with a back light is general, a-Si material constituting the above TFT easily responds to light as is apparent that the material is used for a solar battery, the OFF-state current is increased by irradiation of light (Japanese Patent Application Laid-Open No. 61-145869).
To solve such problem caused by irradiation of light beams, it is proposed to widen the band gap of the a-Si semiconductor film of the TFT or to cover the whole surface of the a-Si semiconductor film with a light-shielding film. However, it is inevitable that the former proposition substantially results in deteriorating the quality of the a-Si semiconductor film and the latter results in lowering the manufacturing yield rate because of an additional process for forming the light-shielding film.